The present embodiments relate to wireless communications systems and are more particularly directed to distinguishing actual cell bit sequences from likely false cell bit sequences.
Wireless communications have become very prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (“CDMA”). In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a “cell.” More particularly, CDMA systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected user station within the cell to determine the proper recipient of a data signal.
CDMA continues to advance along with corresponding standards that have brought forth a next generation wideband CDMA (“WCDMA”). WCDMA includes alternative methods of data transfer, one being frequency division duplex (“FDD”) and another being time division duplex (“TDD”). The present embodiments have particular benefit in TDD and, thus, it is further introduced here. TDD data are transmitted as quadrature phase shift keyed (“QPSK”) symbols in data packets of a predetermined duration or time slot. Within a data frame having 15 of these slots, bidirectional communications are permitted, that is, one or more of the slots may correspond to communications from a base station to a user station while other slots in the same frame may correspond to communications from a user station to a base station.
Each TDD data packet includes a predetermined training sequence in the time slot, referred to in the art as a midamble, where this training sequence represents a known data pattern used for channel estimation. Specifically, the midamble includes information that is unique to a given cell and is selected from a pre-defined set of 128 different possible bit sequences; thus, the unique sequence is assigned to the given cell and that information is encoded within the midamble of data frames transmitted by stations within the corresponding cell. Conversely, data packets exchanged with respect to an adjacent cell have midambles with a different one of the bit sequence sets encoded therein and corresponding to the adjacent cell. Lastly, note that a base station in some instances may communicate with different cells (i.e., is “sectorized”); in this case, then a different and unique midamble is encoded within the data frames communicated with respect to that base station for each of the different cells. For the sake of simplicity in the remainder of this document, each base station is associated only with a single cell and, thus, in this respect, the cell's unique midamble also may be viewed as unique per the corresponding base station. In any event, the basic midamble code may be one of two lengths, where currently these lengths are described as a long basic midamble code with 456 bits and a short basic midamble code with 192 bits. While a basic midamble code consists of the same bit sequence used for communications between a base station and all user stations in the cell corresponding to that base station, each user station in the cell is distinguishable from the others because it is assigned a different time-shifted version of the basic midamble code. The assigned shifting is defined in terms of an offset in the basic midamble code, that is, each user station within the cell is assigned its own offset that represents the amount of time shift adjustment for the user station's basic midamble code. For example, with a short length midamble code thereby having a length of 192 bits, and with eight user stations communicating with one base station, the offsets for each of the eight user stations may be spaced 24 chips apart. Thus, the same basic sequence is used for all of these user stations except that for each user station it is circularly shifted by a different multiple of 24 chips to correspond to the offset of the particular user station. After the circularly-shifted basic sequences are summed, a cyclic prefix is inserted to form a midamble of length 256 chips for a short basic midamble code and of length 512 chips for a long basic midamble code.
Two related aspects arise in connection with midambles, as explained here according to the prior art and as further addressed later in connection with the preferred embodiments. The first aspect is channel estimation and the second aspect is delay profile estimation (“DPE”). Looking first to channel estimation, in the prior art signal paths are received by a receiver at different times, referred to as different chip positions. In response to each of these paths falling within a defined time period referred to as a channel estimation window, and more particularly in response to the midamble in each path in the channel estimation window, the receiver computes a corresponding channel estimate for each path. The channel estimates may be computed using a Fourier transform applied to the entire composite signal that exists in the channel estimation window, where the composite signal is therefore a function of the midambles of any paths occurring within the window. The result of the Fourier transform presents channel estimates at each of the chip positions within the window, and the computed channel estimates are stored with references to the chip position for each estimate. Given the channel estimates, the receiver also performs DPE by noncoherently averaging channel estimates derived from the midambles over many frames, which generally therefore sums the respective absolute channel estimate values for each bit position within the channel estimation window. The DPE therefore represents the average power at each bit position over many frames, thereby attempting to average over the fades and the noise. In response to the DPE, the channel estimates corresponding to those bit positions having an average power greater than some threshold are further used by the receiver for additional signal processing, such as for developing channel estimates using a maximal ratio combining (“MRC”) process.
To further appreciate the context of the preferred embodiments, an additional introduction is made with respect to receipt by a receiver of both actual paths and false paths. Specifically, recall that a midamble used in one cell is different from the midamble used in an adjacent cell. Nonetheless, a receiver may often receive paths from both a station in the cell in which the receiver is located and also paths from other stations in one or more adjacent cells. Thus, in each of the received paths, there is included either a midamble for the cell in which the receiver is located or a midamble from a different cell. Ideally, when the receiver is attempting to communicate only with other stations in the cell in which the receiver is located, then to properly determine its channel estimates it should make that determination only in response to the paths (and their corresponding midambles) from those other stations in the cell in which the receiver is located; thus, these paths are referred to as actual paths. Also in the ideal case, the receiver should disregard those paths received from transmitters of other cells, and those paths are referred to as false paths in that they represent information to the receiver that is not from the cell with which the receiver is attempting to communicate.
While the preceding aspects of channel estimates and DPE have provided a certain level of receiver performance in the prior art, it has been determined in connection with the present inventive embodiments that such operations may be improved, thereby also increasing the performance of additional operations (e.g., MRC) that rely upon these preceding operations. Specifically, it now may be noted that by identifying the paths having a relatively high average power, the DPE is in effect attempting to identify only the actual paths received, while thereby assuming that the paths having a relatively low average power are false paths. However, the present inventors have recognized that while the DPE process will eliminate some false paths which occur due to noise or fading, the DPE process may not eliminate a considerable number of other false paths. Further, this failure of the DPE has been observed to arise due to the high cross-correlation between the different midamble sequences. In other words, for the 128 different possible sequences (of either 192 or 456 bits), there is a considerable cross-correlation between various pairs of these sequences. This cross-correlation will therefore cause false paths to appear, and it is not accommodated by the prior art DPE process. As a result, in the prior art some of these false paths are accepted as actual paths by the DPE process and, thus, the channel estimates corresponding to these false paths are then used for further processing by the receiver, where such uses thereby deplete resources that are better served for processing actual paths. Again by way of example, the channel estimates corresponding to these false paths may be assigned to different fingers in a rake receiver performing MRC analysis, where those fingers would be better suited for assignment to actual path channel estimates. Thus, in the prior art there is a considerable chance for communications from adjacent cells to diminish the ability of a user station to perform its channel estimation.
In view of the preceding, there is a need to improve the DPE in response to actual and false midamble basic sequences, and this need is addressed by the preferred embodiments as described below.